1. Field of the Invention
The present invention relates to a power control unit in a hybrid vehicle having an engine which generates power to be transmitted to a driving wheel and a motor which adds power to the power generated by the engine in a superimposed manner to assist the engine.
2. Description of the Related Art
In recent years, hybrid vehicles which combine an internal combustion engine and an electric motor to generate power and transmit it to driving wheels have been developed and commercialized for reduction in fuel consumption and gas emissions.
In a hybrid vehicle, since the generated torque can be varied easily by controlling electric power to the motor, a device which uses this feature to give a rider a driving feeling which he/she desires has been proposed in JP-A No. 58295/1997 (Japanese Laid-Open Patent Publication No. 9-58295). In this device, three control modes are available where different torque amounts are generated in response to the amount of accelerator operation; in the first control mode, the torque increases gradually in response to the amount of accelerator operation; in the second control mode, the torque increases sharply; and in the third control mode, the torque increases in a manner expressed by a quadratic curve. The rider selects one of these control modes operating a control mode selector switch and the selected control mode is executed under the influence of a prescribed control part.
When the second control mode is selected, the torque increases largely in response to change in the amount of accelerator operation and thus a high response driving feeling is realized.
However, for acceleration in an actual vehicle driving situation, the rider may not only increase the absolute amount of accelerator operation but also intuitively increase the rate of change in the amount of accelerator operation, namely the accelerator operation speed. This is because the rider instinctively feels that a higher acceleration speed will be obtained by operating the accelerator quickly. However, in the device as described in JP-A No. 58295/1997, even if only the operation speed is increased, the driving feeling does not change and the acceleration speed which the rider desires cannot be obtained.
In the device described in JP-A No. 58295/1997, in order to change the driving feeling, the rider must read a manual or the like to understand the characteristic of each control mode and operate the control mode selector switch according to the driving condition.
A hybrid vehicle has different driving modes such as an engine driving mode in which it is driven by an engine only, a motor driving mode in which it is driven by a motor only, and a hybrid mode in which the motor assists the engine in driving it, so that the driving mode is switched automatically depending on the running condition or battery charge condition.
In a hybrid vehicle described in JP-A No. 23312/2000 (Japanese Laid-Open Patent Publication No. 2000-23312), a clutch is provided between an engine and a motor and the driving mode is switched by engaging or disengaging the clutch. In other words, in the motor driving mode, the clutch is disengaged and the motor is driven under the influence of a controller and power is transmitted to a driving wheel through a transmission or the like. On the other hand, in the engine driving mode, the clutch is engaged and the engine is driven so that the driving force of the engine is transmitted to the driving wheel for running.
When the driving mode is switched from the motor driving mode to the engine driving mode, if the driving force and rotation speed of the motor do not agree with those of the engine, some engagement noise may be generated by engagement of the clutch. In order to prevent such engagement noise, in the hybrid vehicle as described in JP-A No. 23312/2000, the torque of the engine and the torque of the motor are detected and while the motor absorbs the torque of the engine, the rotation speed of the clutch input shaft is made to agree with that of the clutch output shaft before the clutch is engaged.
In the hybrid vehicle described in JP-A No. 23312/2000, the torque and rotation speed of the engine and motor are controlled, which requires a complicated structure and a troublesome control sequence. Particularly, a torque sensor is generally expensive and complicated. Besides, there is concern that response to driving mode switching may deteriorate as the structure is complicated.
Traction control units which eliminate driving wheel slip which occur at the time of start or acceleration of a vehicle on a low-friction road surface such as a frozen, snowy or muddy road surface and improve the starting or accelerating ability of the vehicle have been known. The problem of driving wheel slip is solved by applying a braking force or decreasing engine output.
As a technique of decreasing engine output in occurrence of slip, JP-A No. 35329/1998 (Japanese Laid-Open Patent Publication No. 10-35329) discloses a traction control unit which stops fuel supply to the engine for driving the wheel. JP-A No. 232697/1996 (Japanese Laid-Open Patent Publication No. 8-232697) discloses a traction control unit which decreases engine output by delaying the time to ignite the engine. In connection with hybrid vehicles, JP-A No. 63392/2001 (Japanese Laid-Open Patent Publication No. 2001-63392) discloses a technique that in a hybrid vehicle in which front wheels are driven by an engine and a drive motor is connected to rear wheels, when a front wheel slips, a generator driven by the engine generates energy to lower the front wheel driving force and the energy generated by the generator is supplied to the drive motor, producing a four-wheel drive mode.
Generally known hybrid vehicle systems are: a “series hybrid system” in which only the motor is used for driving power and the engine is used only to drive the generator to generate electric energy; a “parallel hybrid system” in which both the motor and engine are used to drive the vehicle selectively depending on the running condition, etc.; and a “series parallel system” in which both the series hybrid system and the parallel hybrid system are selectively used depending on the situation or both are used.
Particularly in vehicles which adopt the series hybrid system, it may be difficult to eliminate slip by changeover from a two-wheel drive mode to a four-wheel drive mode as described in the above JP-A No. 63392/2001 because they often use a mechanism that transmits power of the engine and power of the drive motor to a driving wheel through the same drive shaft.
Furthermore, in the traction control disclosed in JP-A No. 35329/1998, in order to stop fuel supply to the engine, a fuel shutoff valve must be provided separately and the system cannot be used when the vehicle is driven by the motor, so another system must be provided for a situation that it is driven by the motor. Similarly, the traction control disclosed in JP-A No. 232697/1996 cannot be used when the vehicle is driven by the motor and another system must be provided for a situation that the vehicle is driven by the motor.